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Nanocluster growth via “graft-onto”: effects on geometric structures and optical properties

Xi Kang ac, Shan Jinb, Lin Xiongd, Xiao Weiac, Manman Zhouac, Chenwanli Qinac, Yong Peid, Shuxin Wang*ac and Manzhou Zhu*abc
aDepartment of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui Province Key Laboratory of Chemistry for Inorganic/Organic Hybrid Functionalized Materials, Anhui University, Hefei, Anhui 230601, P. R. China. E-mail: ixing@ahu.edu.cn; zmz@ahu.edu.cn
bInstitutes of Physical Science and Information Technology, Anhui University, Hefei, Anhui 230601, P. R. China
cKey Laboratory of Structure and Functional Regulation of Hybrid Materials, Anhui University, Ministry of Education, Hefei, 230601, P. R. China
dDepartment of Chemistry, Key Laboratory of Environmentally Friendly Chemistry and Applications of Ministry of Education, Xiangtan University, Xiangtan, Hunan 411105, China

Received 11th November 2019 , Accepted 26th December 2019

First published on 27th December 2019


Atomically precise engineering on the nanocluster surface remains highly desirable for the fundamental understanding of how surface structures of a nanocluster contribute to its overall properties. In this paper, the concept of “graft-onto” has been exploited to facilitate nanocluster growth on surface structures. Specifically, the Ag2(DPPM)Cl2 complex is used for re-constructing the surface structure of Pt1Ag28(SR)18(PPh3)4 (Pt1Ag28, SR = 1-adamantanethiolate) and producing a size-growth nanocluster – Pt1Ag31(SR)16(DPPM)3Cl3 (Pt1Ag31). The grafting effect of Ag2(DPPM)Cl2 induces both direct changes on the surface structure (e.g., size growth, structural transformation, and surface rotation) and indirect changes on the kernel structure (from a fcc configuration to an icosahedral configuration). Remarkable differences have been observed by comparing optical properties between Pt1Ag28 and Pt1Ag31. Significantly, Pt1Ag31 exhibits high photo-luminescent intensity with a quantum yield of 29.3%, which is six times that of the Pt1Ag28. Overall, this work presents a new approach (i.e., graft-onto) for the precise dictation of nanocluster surface structures at the atomic level.


1 Introduction

It has long been nano-scientists' dream to control compositions and structures of nanomaterials at the atomic level. Through the continuous accumulation of synthetic experience and with the help of advanced analytical methods, researchers can now easily tailor the composition and the morphology of metal nanoparticles.1,2 However, it still remains extremely difficult to realize the atomic-level tailoring of specific sites on the nanoparticle surface (for example, adding or deleting one or two metal atoms at a designated position), which is the most relevant to the physical–chemical properties of such nanomaterials.

Nanoclusters (so-called ultrasmall nanoparticles) are an emerging class of promising nanomaterials owing to their atomically precise structures and intriguing properties.3,4 Because of the quantum size effect of nanoclusters, any perturbations on compositions/structures may induce tremendous variations on clusters' properties.3–6 Knowledge and understanding of these structure–property correlations are keys to the ultimate goal in the nanocluster science – arbitrarily dictating the properties via precisely tailoring the structures. Exploring effective approaches to exquisitely tailor the size, structure, and composition of atomically precise nanoclusters is a prerequisite to achieve this goal.

In recent years, ligand engineering has served as an efficient approach to convert structures of nanoclusters.7 In general, great structural transformation occurs when the peripheral ligands of nanoclusters are drastically substituted by the introduced ligands (for instance, from mono-icosahedral Au11(PPh3)3Cl3 to bi-icosahedral Au25(PPh3)10(SR)5Cl2,7a from Ag44(SR)30 with a hollow kernel to Ag25(SR)18 with a nonhollow kernel,7b and so on). However, it still remains challenging to tailor specific sites on the nanocluster surface without constructing the overall structure. A new approach for tailoring the nanocluster surface is highly desirable for the fundamental understanding of how surface structures in a nanocluster contribute to its overall properties.

By noting two facts about nanoclusters that (i) it is hard to substitute thiol ligands on the nanocluster surface by the introduced phosphorus ligands (because the metal–S covalent bond is more robust than the metal–P coordination bond) and (ii) several thiolated nanoclusters are terminally capped by metal–PPh3 units (such as Pt1Ag28(SR)18(PPh3)4, Ag29(SSR)12(PPh3)4, and Ag33(SR)24(PPh3)4 clusters with Ag–PPh3 terminals),8 we perceive a good opportunity to re-construct the nanocluster surface without largely affecting its whole structure – substituting these terminal metal–PPh3 units by bidentate phosphorus metal complexes. Such a substitution may not only fine-tune the nanocluster surface structure, but also shed light on structure–property correlations at the atomic level.

In the current work, we report a “graft-onto” strategy to facilitate a controllable size-growth of the nanocluster surface. Induced by the addition of the Ag2(DPPM)Cl2 complex (DPPM = bis-(diphenylphosphino)-methane), Pt1Ag28(SR)18(PPh3)4 (Pt1Ag28; SR = 1-adamantanethiolate) converts into a size-growth nanocluster – Pt1Ag31(SR)16(DPPM)3Cl3 (Pt1Ag31). Great changes (including size growth, structural transformation, and surface rotation) take place on the outermost shell of Pt1Ag28, owing to the direct grafting effect of the Ag2(DPPM)Cl2. The changes on the outermost shell further induce the transformation of the innermost Pt1Ag12 kernel from a fcc configuration in Pt1Ag28 to an icosahedral configuration in Pt1Ag31. Pt1Ag28 and Pt1Ag31 nanoclusters reflect remarkable differences in both optical absorption and PL emission. Significantly, Pt1Ag31 displays high photo-luminescence (PL) intensity with a quantum yield (QY) of 29.3%, which is six times that of the Pt1Ag28 (PL QY = 4.9%).

2 Experimental methods

Materials

All reagents were purchased from Acros Organics and Sigma-Aldrich and used without further purification: hexachloroplatinic(IV) acid (H2PtCl6·6H2O, 99%, metals basis), silver nitrate (AgNO3, 99% metals basis), adamantane-1-thiol (C10H15SH, HS-Adm, 95%), triphenylphosphine (PPh3, 95%), bis(diphenylphosphino)methane ((C6H5)2PCH2P(C6H5)2, DPPM, 98%), sodium borohydride (NaBH4, 99.9%), sodium chloride (NaCl, 99.5%), sodium hexafluoroantimonate (NaSbF6, 99%), rhodamine B (RB, for fluorescence), methylene chloride (CH2Cl2, HPLC, Aldrich), methanol (CH3OH, HPLC, Aldrich), ethyl acetate (CH3COOC2H5, HPLC, Aldrich), ethanol (CH3CH2OH, HPLC, Aldrich), ether (C2H5OC2H5, HPLC, Aldrich), and 2-methyltetrahydrofuran (C4H7O-2-CH3, HPLC, Aldrich).

Synthesis of the [Pt1Ag28(S-Adm)18(PPh3)4]Cl2 nanocluster

For the nanocluster synthesis, AgNO3 (29 mg, 0.17 mmol) and H2PtCl6·6H2O (5 mg, 0.01 mmol) were dissolved in CH3OH (5 mL) and CH3COOC2H5 (35 mL). The solution was vigorously stirred (1200 rpm) with magnetic stirring for 15 min. Then, Adm-SH (0.1 g) and PPh3 (0.1 g) were added and the reaction was vigorously stirred (1200 rpm) for another 90 min. After this, NaBH4 aqueous solution (1 mL, 20 mg mL−1) was added quickly to the above mixture. The reaction was allowed to proceed for 36 h under a N2 atmosphere. After this, the aqueous layer was removed, and the mixture in the organic phase was rotavaporated under vacuum. Then approximately 30 × 3 mL of CH3CH2OH was used to wash the obtained nanoclusters. The precipitate was dissolved in CH2Cl2, which produced the [Pt1Ag28(S-Adm)18(PPh3)4]Cl2 nanocluster. The yield is 45% based on the Ag element (calculated from AgNO3) for the synthesis of [Pt1Ag28(S-Adm)18(PPh3)4]Cl2.

Synthesis of the Ag2(DPPM)Cl2 complex

0.17 g of AgNO3 (1 mmol) was dissolved in 50 mL of CH3CH2OH, and NaCl aqueous solution (6 mL, 10 mg mL−1) was added quickly to the above mixture. The reaction was stirred for 1 minute. The white precipitate was collected and rotavaporated under vacuum, which produced the AgCl powder. Then, 0.07 g of AgCl (0.5 mmol) was dispersed in 20 mL of CH2Cl2, to which solution 0.19 g DPPM was added. The reaction was vigorously stirred (1200 rpm) with magnetic stirring for 30 minutes. After this, the solution was evaporated to dryness, which produced the Ag2(DPPM)Cl2 complex. The yield is about 95% based on the Ag element (calculated from AgCl) for the synthesis of Ag2(DPPM)Cl2.

Synthesis of the [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4 nanocluster

For the nanocluster synthesis, 30 mg of [Pt1Ag28(S-Adm)18(PPh3)4]Cl2 was dissolved in 30 mL of CH2Cl2, to which 10 mg of Ag2(DPPM)Cl2 was added. The reaction was allowed to proceed for 10 minutes at room temperature. After this, the organic layer was separated from the precipitate and evaporated to dryness. Then, approximately 30 × 3 mL of CH3CH2OH was used to wash the obtained nanoclusters. The precipitate was dissolved in CH2Cl2, which produced the [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4 nanocluster. The yield is about 85% based on the Ag element (calculated from the Pt1Ag28) for the synthesis of [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4.

Single-crystal growth of [Pt1Ag31(S-Adm)16(DPPM)3Cl3](SbF6)4

For accelerating the crystallization process and improving the quality of crystals, the counterion Cl in [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4 was replaced by SbF6. The reaction equation is [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4 + 4 SbF6 → [Pt1Ag31(S-Adm)16(DPPM)3Cl3](SbF6)4 + 4Cl. Specifically, 20 mg of [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4 was dissolved in 20 mL of CH2Cl2. Then, 1 mL of NaSbF6–CH3CH2OH solution (5 mg mL−1) was added. After 3 minutes, the organic layer was separated from the precipitate and evaporated to dryness. The precipitate was dissolved in CH2Cl2, which produced the [Pt1Ag31(S-Adm)16(DPPM)3Cl3](SbF6)4 nanocluster. Nanoclusters were crystallized in a CH2Cl2/ether system with a vapor diffusion method. Specifically, 20 mg of clusters was dissolved in 5 mL of CH2Cl2, and the obtained solution was then vapor diffused using 50 mL of ether. After 3 days, dark red crystals of Pt1Ag31 were collected and subjected to X-ray diffraction to determine the structure. The CCDC number of [Pt1Ag31(S-Adm)16(DPPM)3Cl3](SbF6)4 is 1937755. Notably, the optical absorption and PL emission properties of the Pt1Ag31 nanocluster remain unchanged after the counter-ion replacement.

Test of the temperature-photoluminescence (PL) intensity correlation

The nanocluster (0.1 mg) was dissolved in 5 mL of the CH2Cl2/C4H7O-2-CH3 (v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]1) mixture. Then, the solutions were cooled from 293 K to different temperatures and the PL spectra were measured.

X-ray crystallography

The data collection for single crystal X-ray diffraction was carried out on a Bruker Smart APEX II CCD diffractometer under a nitrogen flow at 170 K, using graphite-monochromatized Mo Kα radiation (λ = 0.71073 Å). Data reductions and absorption corrections were performed using the SAINT and SADABS programs, respectively.9a The electron density was squeezed using PLATON, and detailed information can be found in Table S3. The structure was solved by direct methods and refined with full-matrix least squares on F2 using the SHELXTL software package.9b All non-hydrogen atoms were refined anisotropically, and all the hydrogen atoms were set in geometrically calculated positions and refined isotropically using a riding model.

Theoretical methods

Density functional theory (DFT) calculations were employed to optimize the geometric structures and calculated the Kohn–Sham orbitals of Pt1Ag28 and Pt1Ag31 nanoclusters using the Perdew–Burke–Ernzerhof (PBE) GGA functional.10a The triple-zeta polarized (TZP) basis set with inclusion of the scalar relativistic effect via a zeroth-order regular approximation (ZORA) implemented in the ADF package was adopted.10b

Characterization

All UV-vis absorption spectra of the nanoclusters dissolved in CH2Cl2 were recorded using an Agilent 8453 diode array spectrometer, whose background correction was made using a CH2Cl2 blank.

Photo-luminescence (PL) spectra were measured on an FL-4500 spectrofluorometer with the same optical density (OD) of 0.05. In these experiments, the nanocluster solutions were prepared in CH2Cl2 at a concentration of less than 1 mg mL−1.

Absolute quantum yield (QY) was measured with dilute solutions of nanoclusters on a HORIBA FluoroMax-4P. For determining the QYs of clusters, the nanocluster solutions were prepared in CH2Cl2 with the same OD of 0.05. Besides, the PL comparison between the Pt1Ag31(S-Adm)16(DPPM)3Cl3 nanocluster and rhodamine B was performed, to further determine the PL QY of the Pt1Ag31(S-Adm)16(DPPM)3Cl3 nanocluster.

Thermogravimetric analysis (TGA) was carried out on a thermogravimetric analyzer (DTG-60H, Shimadzu Instruments, Inc.). 10 mg of clusters was used for collecting the TGA data on clusters.

X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 configured with a monochromated Al Kα (1486.8 eV) 150 W X-ray source, 0.5 mm circular spot size, a flood gun to counter charging effects, and analysis chamber base pressure lower than 1 × 10−9 mbar.

Inductively coupled plasma-atomic emission spectrometry (ICP-AES) measurements were performed on an Atomscan advantage instrument from Thormo Jarrell Ash Corporation (USA).

Elemental analysis (EA) was performed on Vario EL cube. 3 mg of each cluster sample was used for collecting the EA data.

Energy-dispersive X-ray spectroscopy (EDS) analyses were performed on a JEOL JEM-2100F FEG TEM operated at 200 kV. Nanocluster powder samples were used for the analysis.

Electrospray ionization time-of-flight mass spectrometry (ESI-TOF-MS) measurements were performed using a MicrOTOF-QIII high-resolution mass spectrometer; for preparing the ESI sample, the clusters were dissolved in CH2Cl2 (1 mg mL−1) and diluted (v/v = 1[thin space (1/6-em)]:[thin space (1/6-em)]2) with methanol.

3 Results and discussion

Pt1Ag28 was prepared first via an in situ synthetic procedure (see the Experimental section for more details). A combination of ESI-MS, UV-vis absorption, PL, XPS, ICP, and EA results unambiguously identified that the obtained Pt1Ag28 nanocluster is the same as the one reported previously (Fig. S1, and Tables S1, S2).8h The EDS results demonstrated the presence of Cl in the cluster system, which was considered as the counterion for the Pt1Ag28 nanocluster, namely, [Pt1Ag28(S-Adm)18(PPh3)4]Cl2 (Fig. S2).

The reaction between Pt1Ag28 and Ag2(DPPM)Cl2 generates a Pt1Ag31 nanocluster, wherein the Ag–PPh3 vertexes were substituted by the Ag2(DPPM)Cl, accompanied by a size-growth of the metallic kernel from M29 to M32 (M = Pt/Ag; Fig. 1). As for the overall structure, the Ag–PPh3 terminals (in Pt1Ag28) are bonded onto the nanoclusters with an I-type growth mode, whereas the Ag2(DPPM)Cl terminals (in Pt1Ag31) follow a Y-type growth mode (Fig. 1). The transformation of the nanocluster terminal from the single-linked Ag–PPh3 (I type; see Fig. 1A) into the double-linked Ag2(DPPM)Cl (Y type; see Fig. 1B) reflects the “graft-onto growth” and accounts for the size growth, structural transformation, and surface rotation of the nanocluster.


image file: c9sc05700e-f1.tif
Fig. 1 Transformation from Pt1Ag28 into Pt1Ag31. (A) Schematic illustration of the I-type growth of the Ag–PPh3 terminal and the structure of Pt1Ag28. (B) Schematic illustration of the Y-type growth of the Ag2(DPPM)Cl terminal and the structure of Pt1Ag31. In this transformation process, the Ag–PPh3 terminals in Pt1Ag28 are peeled off, and the Ag2(DPPM)Cl terminals are introduced. Color legends: dark green sphere, Pt; blue sphere, Ag; red sphere, S; purple sphere, P; green sphere, Cl; grey sphere, C. For clarity, all H atoms and some C atoms are omitted.

The ESI-MS and EDS results identified the molecular formula as [Pt1Ag31(S-Adm)16(DPPM)3Cl3]Cl4 (Fig. S3 and S4). Both Pt1Ag28 and Pt1Ag31 clusters contain 8 free valence electrons – (i) for Pt1Ag28, the free electron count is 28(Ag) − 18(SR) − 2(charge) = 8; (ii) for Pt1Ag31, the free electron count is 31(Ag) − 16(SR) − 3 (Cl) − 4(charge) = 8. The atomic ratio of Pt to Ag in Pt1Ag31 was analyzed by XPS and ICP, and the experimental results were consistent with the theoretical ratio (Fig. S5, and Table S1). TGA of the Pt1Ag31 nanocluster showed a total weight loss of 52.65%, matching the theoretical value of 53.54% (the proportion of SR, DPPM, Cl ligands and the Cl counterion in the overall formula; Fig. S6).

The structural comparison between Pt1Ag28 and Pt1Ag31 is shown in Fig. 2 (see Fig. S7 for the total structure of Pt1Ag31). Pt1Ag28 comprises a fcc Pt1Ag12 kernel, a trilateral Ag12(SR)15(PPh3)3 shell, and a helical Ag4(SR)3(PPh3)1 unit (Fig. 2A–F). The trilateral Ag12(SR)15(PPh3)3 shell is constituted by assembling of three same Ag4(SR)6(PPh3)1 units (Ag3(SR)6 face + Ag–PPh3 terminals) by sharing the terminal thiol ligands (Fig. 2B and M).


image file: c9sc05700e-f2.tif
Fig. 2 Structural anatomies of Pt1Ag28 and Pt1Ag31 nanoclusters. (A–F and M) Structural anatomy of the Pt1Ag28 nanocluster. (G–L and N) Structural anatomy of the Pt1Ag31 nanocluster. Color legends: dark green sphere, Pt; blue/dark grey/orange/dark blue sphere, Ag; brown sphere, motif-shared Ag; red sphere, S; pink sphere, motif-shared S; purple sphere, P; green sphere, Cl; grey sphere, C. For clarity, all H atoms and some C atoms are omitted.

With the grafting effect, the vertex Ag-PPh3 units on the Pt1Ag28 surface are peeled off, and Ag2(DPPM)Cl units are introduced. As a result, three additional Ag atoms (Pt1Ag31Pt1Ag28 = 3 Ag) are incorporated onto the nanocluster surface because of the grafting effect of bidentate DPPM ligands. Three Cl ligands are also introduced to further stabilize the surface structure of Pt1Ag31. More specifically—

(i) Although the composition of the Pt1Ag12 kernel maintains throughout the graft-onto process, the fcc configuration of the Pt1Ag12 kernel in Pt1Ag28 alters to an icosahedral configuration in Pt1Ag31 (Fig. 2A and G). The average bond length between Pt(core) and Ag(kernel surface) in Pt1Ag31 is smaller than that in Pt1Ag28, whereas the bonds between Ag(kernel surface) and Ag(kernel surface) in Pt1Ag31 are much longer than those in Pt1Ag28 (Table 1 and Fig. S8).

Table 1 Comparison of bond lengths in Pt1Ag28 and Pt1Ag31 nanoclusters. Such bonds are highlighted in Fig. S8
  Pt1Ag28 Pt1Ag31 Diff.
Pt(core)–Ag(kernel surface) bond 2.768–2.797 Å (Avg. 2.838 Å) 2.735–2.786 Å (Avg. 2.760 Å) −2.83%
Ag(kernel surface)–Ag (kernel surface) bond 2.751–2.848 Å (Avg. 2.802 Å) 2.817–3.144 Å (Avg. 2.906 Å) +3.58%
Ag(kernel surface)–S(motif) bond 2.438–2.498 Å (Avg. 2.472 Å) 2.445–2.591 Å (Avg. 2.495 Å) +0.92%
Ag(motif)–S(motif) bond 2.254–2.992 Å (Avg. 2.560 Å) 2.356–2.835 Å (Avg. 2.460 Å) +4.07%
Ag(motif)–P(vertex) bond 2.292–2.384 Å (Avg. 2.356 Å) 2.397–2.428 Å (Avg. 2.405 Å) +2.04%


(ii) The trilateral Ag15(SR)13(DPPM)3Cl3 shell in Pt1Ag31 is constituted by the assembly of three same Ag6(SR)6(DPPM)1Cl1 units via sharing Ag2(SR)3 edges (Fig. 2H and N). Due to the steric hindrance effect, only 13 thiol ligands exist in the trilateral Ag15(SR)13(DPPM)3Cl3 shell and less than the 15 thiol ligands in the Ag12(SR)15(PPh3)3 trilateral shell of Pt1Ag28 (Fig. 2B and H). In each Ag6(SR)6(DPPM)1Cl1 unit, the Cl ligand fixes two Ag atoms that bond with the DPPM ligand (Fig. 2N). All Ag-ligand interactions (including Ag(kernel surface)–S(motif), Ag(motif)–S(motif), and Ag(motif)–P(vertex)) in Pt1Ag31 are longer than those in Pt1Ag28 (Table 1).

(iii) The Pt1Ag24(SR)15(PPh3)3 structure in Pt1Ag28 is covered by a helical Ag4(SR)3(PPh3)1 unit (Fig. 2D), whereas the corresponding structure in Pt1Ag31 is just Ag4(SR)3 (Fig. 2J); that is, the terminal PPh3 ligand is peeled off. A similar situation has recently been observed in the transformation of Ag29(SSR)12(PPh3)4 into Cs3Ag29(SSR)12(DMF)x.11 For the Cs3Ag29(SSR)12(DMF)x nanocluster, because of the absence of the vertex PPh3 ligand, the terminal Ag atom becomes closer to the innermost Ag13 kernel.11 A similar situation has been observed in this work – the average distance between the terminal Ag and adjacent Ag atoms in the innermost Pt1Ag12 kernel in Pt1Ag31 is 4.026 Å, which is much shorter than that in Pt1Ag28 (4.290 Å, as shown in Fig. S9). In this context, the terminal Ag-based structure in Pt1Ag31 becomes more contractive for reducing the exposure of this bare Ag atom, which in turn makes the overall structure more robust.

Collectively, the “graft-onto” process on the Pt1Ag28 surface changes the vertex structure from PPh3-Ag to DPPM-Ag2-Cl, resulting in the size-growth and surface structural transformation of the nanocluster. The transformation of the outermost shell further induces changes on kernel–shell interactions, and such changes alter the innermost Pt1Ag12 kernel from a fcc configuration in Pt1Ag28 to an icosahedral configuration in Pt1Ag31.

From the structural point of view, aside from Pt1Ag28(SR)18(PPh3)4, several other metal nanoclusters are terminally capped by metal–PPh3, such as Ag29(SSR)12(PPh3)4, Ag33(SR)24(PPh3)4, Au23(PhC[triple bond, length as m-dash]C)9(PPh3)6, Au24(PhC[triple bond, length as m-dash]C)14(PR)4, and so on.8 Our reported “graft-onto” strategy might also be applicable in these cluster systems for controlling their surface structures. Future work will focus on extending the “graft-onto” strategy to other cluster systems.

Both Pt1Ag28 and Pt1Ag31 nanoclusters are stable in DMF at 50 °C for at least 24 hours (Fig. S10A and C). At 80 °C, the optical absorptions of Pt1Ag28 disappear over time (Fig. S10B); by comparison, the Pt1Ag31 nanocluster is stable enough to maintain its optical absorptions (Fig. S10D). We propose that the enhanced thermal stability of Pt1Ag31 results from its more robust structure – compared with PPh3, the introduced DPPM ligands have more ability to fix the nanocluster surface and thus suppress the vibration of the overall structure.

The optical properties of Pt1Ag28 and Pt1Ag31 nanoclusters are compared. Optical absorption of Pt1Ag28 shows an intense peak at 445 nm and a shoulder peak at 540 nm. The transformation of Pt1Ag28 into Pt1Ag31 results in an obvious blue-shift for each peak – the peak at 445 nm blue-shifts to 430 nm and becomes wider, and the shoulder band at 540 nm blue-shifts to 525 nm (Fig. 3A). The blue shift of the maximum optical absorption of nanoclusters (i.e., from 540 nm of Pt1Ag28 to 525 nm of Pt1Ag31) always represents the enlargement of the HOMO–LUMO energy gap (HOMO: the highest occupied molecular orbital; LUMO: the lowest unoccupied molecular orbital), which matches the DFT calculation results that Pt1Ag31 displays a larger energy gap relative to Pt1Ag28 (1.92 eV versus 1.76 eV, Fig. S11).


image file: c9sc05700e-f3.tif
Fig. 3 Optical properties of nanoclusters. (A) Optical absorptions of Pt1Ag28 and Pt1Ag31 nanoclusters. (B) PL emissions of Pt1Ag28 and Pt1Ag31 nanoclusters. Insets in B: peak shift in normalized PL spectra, and digital photo of each cluster in CH2Cl2 under UV light.

The Pt1Ag28 nanocluster (in CH2Cl2 solution) emits at 672 nm, with a photo-luminescence quantum yield (PL QY) of 4.9%.8h By comparison, the Pt1Ag31 nanocluster (in CH2Cl2 solution, with the same OD as Pt1Ag28 solution) emits at 651 nm, representing a 21 nm blue shift relative to that of the Pt1Ag28. Significantly, the PL QY of Pt1Ag31 in CH2Cl2 is 29.3%; that is, the PL intensity of Pt1Ag31 is six times that of the Pt1Ag28. Such an enhancement can also be inferred from the PL spectra of two nanoclusters (Fig. 3B). Besides, the PL comparison between Pt1Ag31 and rhodamine B further determined the PL QY of the nanocluster (Fig. S12 and S13). Under weak UV light, the emission of Pt1Ag28 is hard to be observed, whereas the PL of Pt1Ag31 is strong enough to be perceived with the naked eye (Fig. 3B, insets). Such a PL enhancement may result from the enhanced stabilization ability of DPPM relative to PPh3 – the introduced DPPM ligand firmly fixes the surface structure and suppresses the vibration of the overall structure; in this context, the energy dissipation of the photo-excited Pt1Ag31 reduces with non-radiative transitions (mainly affected by intramolecular vibrations), but enhances with radiative transitions (through luminescence).

Temperature-dependent fluorescence of the Pt1Ag31 nanocluster was monitored. For the Pt1Ag28 nanocluster, our previous work demonstrated a 20-fold enhancement on the PL intensity of Pt1Ag28 (in DMF solution) along with the temperature-lowering process from 293 K to 125 K, and the PL QY increased from 9.3% to ∼100% (Fig. S14).12a As to the Pt1Ag31 nanocluster (Fig. 4), the PL intensity presented a 4.5-fold enhancement by comparing the 179 K data with the 293 K data (Fig. 4A–C), and the optical absorption just exhibited a 1.3-fold enhancement in the corresponding temperature-lowering process (Fig. 4D). Accordingly, the PL QY of Pt1Ag31 was almost 100% when the temperature was lower than 179 K. Such an enhancement of PL intensity resulted from the reduced energy consumption of thermal vibrations of nanoclusters (non-radiative transition) reduced by the reduced temperature; in this context, the excitation energy could only be released by the PL approach (radiative transition).11,12


image file: c9sc05700e-f4.tif
Fig. 4 Temperature-dependent PL of Pt1Ag31 (dissolved in CH2Cl2). (A) Temperature-dependent emission of Pt1Ag31. (B) The PL intensity of Pt1Ag31 at the fixed point of 651 nm. (C) The derivative results for the temperature-dependent PL intensity of Pt1Ag31. (D) Optical absorption of Pt1Ag31 at 293 K and 179 K.

4 Conclusions

In summary, a “graft-onto” strategy is presented for facilitating a controllable size-growth of the nanocluster surface. The addition of the Ag2(DPPM)Cl2 complex converts Pt1Ag28(S-Adm)18(PPh3)4 into a size-growth nanocluster, namely, Pt1Ag31(S-Adm)16(DPPM)3Cl3. Induced by the grafting effect, direct changes on the surface structure (e.g., size growth, structural transformation, and surface rotation) and indirect changes on the kernel structure (from a fcc configuration to an icosahedral configuration) take place. Obvious differences have been observed by comparing the optical properties (optical absorption and PL emission) of two nanoclusters. Significantly, Pt1Ag31(SR)16(DPPM)3Cl3 displayed a high PL intensity with a PL QY of 29.3%, which is six times that of the Pt1Ag28(SR)18(PPh3)4. Our work presents a new strategy for controllably re-constructing the nanocluster surface at the atomic level, which hopefully sheds light on the fundamental understanding of how surface structures in a nanocluster contribute to its overall properties.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge the financial support by the NSFC (U1532141, 21631001, 21871001, and 21803001), the Ministry of Education, the Education Department of Anhui Province (KJ2017A010), and 211 Project of Anhui University.

Notes and references

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Footnotes

Electronic supplementary information (ESI) available: Fig. S1–S14 and Tables S1–S3 for the EDS, ESI-MS, XPS, ICP, TGA, and stability and PL results of nanoclusters, and the structural comparison between nanoclusters. CCDC 1937755. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9sc05700e
These authors contributed equally to this work.

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